1. Field of the Invention
This invention relates to solder mask coatings having improved performance characteristics, and, more particularly it is concerned with aqueous alkaline developable, UV curable compositions for making flexible solder mask coatings having excellent electrical insulation resistance under extreme testing conditions.
2. Description of the Prior Art
A number of U.S. patents disclose photopolymerizable prepolymers and compositions for use as solder masks in printed circuit boards, including U.S. Pat. No. 3,660,088; 3,753,720; 3,883,352; 3,887,450; 4,199,163; 4,361,640; 4,436,806; 4,458,007; 4,506,004; and 4,508,916. However, none of these UV curable compounds and compositions are entirely satisfactory for use as a commercial solder mask.
A feature of a preferred solder mask coating is an ability to be developed with an aqueous, slightly alkaline solution, thereby avoiding the use of organic solvents in the developing step. The desired solder mask coatings also should exhibit flexibility, heat and chemical resistance, surface hardness, abrasion resistance, adhesion to the underlying metal of the printed circuit board, and a high cure depth at low dosages. Particularly sought after are solder mask which exhibit excellent electrical insulation resistance and a rapid curing rate. These stringent requirements imply that the structure of the UV curable compound and its compositions must be carefully designed in order that the solder mask can achieve commercial acceptability.
For example, in Rendulic, U.S. Pat. No. 4,436,806, there is described a method and apparatus for making a printed circuit board by imaging a liquid polymer which has been coated onto the board by roller coating. In this process, imaging is carried out while the polymer is wet, the photo tool being positioned in a close relationship with the coated board. The liquid polymer compositions disclosed by Rendulic for use in this process are urethane acrylates having a polyether or polyester extending chain unit. These compositions have a very low viscosity which is desirable for forming coatings with a roller coater. However, these compositions have a relatively low degree of ethylenic unsaturation per weight unit so that they require a relatively high energy level for curing.
In contrast to the imaging technique of the above-mentioned patent, Sullivan, in U.S. Pat. No. 4,506,004, describes a contact method of imaging a liquid solder mask coating on a printed circuit board. In this method, the UV curable solder mask coating is screen printed to a given thickness, prehardened and imaged by exposure with suitable UV energy. The desired characteristics of good electrical performance and excellent printing resolution are the attributes of the Sullivan system. The requirements of a suitable photopolymerizable compound for use in this process include an ability to form a smooth, flexible coating which can impart chemical and heat resistance to the solder mask. Furthermore, the coating must exhibit excellent adhesion to the board, an ability to be cured to a desired depth with a minimum of UV energy, and excellent electrical insulation resistance.
Accordingly, it is an object of this invention to provide a UV curable composition for making a solder mask having advantageous performance characteristics.
Another object of this invention is to provide a UV curable composition, which includes a mixture of UV curable compounds, for use as a solder mask, and, particularly, a composition which can be developed rapidly in an aqueous, slightly alkaline solution.
A particularly object herein is to provide a flexible solder mask coating which exhibits an electrical insulation resistance of greater than 10.sup.7 ohm-cm. after 7 days exposure, at 50.degree. C. and 90% relative humidity.
Still another object herein is to provide a solder mask coating which can be rapidly cured, preferably at an energy level of less than 4.5 joules per sq. cm., to provide a cure depth of at least 18 mils at this energy level.
Another object of this invention is to provide a UV curable composition which can be coated, suitably by screen printing techniques, to form a smooth, uniform, glossy, flexible, coating up to 3 mil thickness.
Yet another object of the invention is to provide a solder mask composition which is stable without gelation for at least six months.
A further object herein is to provide a flexible solder mask coating having substantially 100% adhesion to the underlying metal of the printed circuit board, particularly on copper and tin-lead.
Another object herein is to provide a solder mask product which exhibits good surface hardness, abrasion, heat and organic solvent resistance.
A specific object herein is to provide a solder mask composition which is particularly adapted for use in the method and apparatus of making printed circuit boards described in U.S. Pat. No. 4,506,004.
Another specific object herein is to provide a mixture of UV curable acrylate compounds which will meet all of the aforesaid objects in an advantageous manner.
These and other objects and features of the invention will be made apparent from the following description of the invention.